1. Field of the Invention
The present invention relates to optical systems. More specifically, the present invention relates to wide field of view, radial scanning sensors.
2. Description of the Related Art
In certain imaging applications such as full earth surveillance from low altitude space platforms, missile launch warning from an airborne platform, or airborne threat detection from a ground base location, an extremely wide field of view (120° or more) optical system is required so that a very large two dimensional region of the object space may be covered and searched in a short period of time. Due to the large amount of spatial information to be collected while covering this large field of view, and due to the state of the art in detector array technology, particularly in the infrared portion of the spectrum, it is desirable in such imaging applications to utilize linear detector array technology. The use of a linear detector array dictates the need for some type of scanning technique and device, which allows the linear array to cover a two dimensional field of view. Further, it is desirable that this scanning technique and device be such that the detector assembly which includes the detector array, electronics and cryogenics, to be located on a stationary portion of the optical system. This latter desire is motivated by the difficulties associated with transferring electrical power and signals and cryogenic connections across a dynamic interface.
The prior art in this area is well represented by U.S. Pat. No. 5,347,391 entitled “Ultra Wide Field Of View Scanning Surveillance Optical System” by L. G. Cook and B. A. Wheeler, which describes a full-field refractive optical system that employs a rotating Pechan prism, with five internal reflections, as a scanning device. While useful, this prior art has several limitations. First, the entire field of view is covered in a single refractive optical system which is stressed to its limits and offers little growth in aperture or field of view. Second, the refractive nature of the telescope containing the scanning Pechan prism makes the coverage of a broad spectral band difficult. Third, the required amount and quality of the silicon material needed to construct the Pechan prism is a cost concern. Also, the effect of imperfections in the construction and alignment of the Pechan prism can significantly impact the sensor line-of-sight, transmission efficiency and image quality as the Pechan is rotated. Finally, the nonrelayed nature of the refractive telescope makes even partial cold shielding (when needed) difficult.
Hence, a need exists in the art for an improved optical system which provides a requisite wide field of view, allows broad spectral coverage and cold shielding, does not require a Pechan prism, and which locates the detector array, cryogenics, and electronics off of the scanning part of the sensor.